1. Field of the Invention
The present invention relates generally to a gas turbine engine, and more specifically to a turbine rotor blade with tip rail cooling.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
In a gas turbine engine, such as a large frame heavy-duty industrial gas turbine (IGT) engine, a hot gas stream generated in a combustor is passed through a turbine to produce mechanical work. The turbine includes one or more rows or stages of stator vanes and rotor blades that react with the hot gas stream in a progressively decreasing temperature. The efficiency of the turbine—and therefore the engine—can be increased by passing a higher temperature gas stream into the turbine. However, the turbine inlet temperature is limited to the material properties of the turbine, especially the first stage vanes and blades, and an amount of cooling capability for these first stage airfoils.
The first stage rotor blade and stator vanes are exposed to the highest gas stream temperatures, with the temperature gradually decreasing as the gas stream passes through the turbine stages. The first and second stage airfoils (blades and vanes) must be cooled by passing cooling air through internal cooling passages and discharging the cooling air through film cooling holes to provide a blanket layer of cooling air to protect the hot metal surface from the hot gas stream.
Turbine blades and vanes make use of combinations of impingement cooling, convection cooling and film cooling to provide cooling and protection from the hot gas stream passing through the turbine. Airfoils with thin airfoil walls can be cooled better than an airfoil with a relatively thick wall because the heat transfer rate through a thin wall is greater than through a thick wall. However, modern turbine blades and vanes are produced using the lost wax or investment casting process in which a mold with a ceramic core is used to form the cooling passages within the metal piece. However, thin walls cannot be formed using the lost wax or investment casting process.
Another problem with turbine rotor blades is the hot gas leakage across the blade tip gap. Because the blade is exposed to different temperatures during engine operation such as a cold state at startup and a hot state at the steady state operation, the blade thermally expands and therefore the tip gap distance will change. The tip gap allows for hot gas leakage to flow between the blade tip and the blade outer air seal or BOAS. This hot gas leakage flow will create excess temperatures for the blade tips and the tip edges if not adequately cooling is available. The resulting hot spots will cause erosion damage that will shorten the blade useful life and decrease the efficiency of the turbine.